The invention relates to circuitry associated with a cathode ray tube for TV receivers and the like for selectively inhibiting the beam current in the tube when the forward sweep portion of the deflection voltage normally applied to such tube is absent.
In TV receivers and particularly those with thyristorinstrumented deflection circuits, there is present in the absence of suitable suppression circuitry a sharply focused light spot on the screen of the cathode ray tube when the forward sweep portion of the deflection voltage is absent, as when the TV set is turned off. This phenomenon is caused by the fact that, although the sweep voltages for the tube are in generally immediately disabled when the set is inactivated, the cathode of the picture tube continues to emit electrons toward the screen for a short time thereafter. Since, because of the large capacitors associated with the high tube potential, such high voltage does not decay immediately, a stationary light spot is formed on the screen unless a suitable negative blocking potential is immediately applied to the brightness control grid by a suppression circuit.
Presently known suppression circuits of this type generate the required negative blocking voltage directly or indirectly from an operating voltage of the TV set which ideally collapses immediately when the set is disabled. Their common disadvantages is that their effectiveness is directly dependent on the rate of decay or collapse of the associated operating voltage, so that particularly in those sets having semiconductor deflection circuits wherein the moving light spot becomes stationary immediately upon set turn-off, the above-mentioned operating voltage decays too slowly to provide an effective inhibiting signal.